Tachometers have been used for many years with variable speed dynamoelectric machines either motors or generators, and perhaps are more commonly used with DC motors which are variable speed devices in order, to provide an accurate indication of the speed of the motor. Quite often this speed signal is returned to an electrical control circuit to correct for any mismatch between the desired and the actual speed. This may be a part of an entire drive system for a large piece of machinery, for example, where it is desired to closely coordinate the speeds of several motors driving different parts of the machinery so that a web, for example, moving through the machinery will not be stretched or broken.
The prior art has known many forms of tachometers, many of which have been constructed as small DC or AC generators in the form of separate dynamoelectric machines with their own bearings and driven in some manner from the shaft of the DC motor, the speed of which is to be sensed. In many cases the DC motor has driven a load at what is termed the back end of the motor and the front end of the motor has been provided with a shaft extension to drive such prior art tachometers. However, there are several occasions when the front end of the motor is not readily available from which to drive such a tachometer. Three such occasions are:
1. Where the DC motor has a double shaft extension, e.g. for tandem motor drives, PA1 2. When a brake may be mounted on the front end of the motor, and PA1 3. When the fan and cover of a totally enclosed fan cooled motor is provided on the front end of such DC motor.
When the front end of the motor is already being used for one of the above-mentioned purposes, then the prior art had difficulty in mounting a tachometer also on such front end of the motor. Such prior art mounting usually took the form of two choices: either a timing belt drive to a laterally mounted tachometer, or else a cantilever mounting of the tachometer with the tachometer driven coaxially from the front end shaft extension. In the former choice, the laterally displaced drive with a timing belt had additional problems of providing a suitable mounting for the tachometer and a suitable drive arrangement with space for the timing belt and pulley. In the latter choice the cantilever coaxial mounting of the tachometer meant that the tachometer increased the length of the motor by a minimum of about five inches and in many cases by as much as twenty-one inches where in combined analog and digital output from the tachometer was desired. In many cases room for such a long tachometer extension was simply not available. In both of these two choices of prior art tachometer mounting, the tachometer was a complete dynamoelectric machine by itself, not only with a rotor and stator but also a frame, end brackets and bearings at both ends of the shaft to support the tachometer rotor. This made an expensive construction. Also in many cases it was necessary to provide a flexible or universal shaft connection because one could not rely on the shaft of the tachometer being exactly coaxial with the shaft of the DC motor. In the prior art type without separate bearings, if the tachometer rotor and stator were not coaxial, then there was the problem of run-out between the tachometer rotor and stator which would give undesirable variations in the tachometer voltage output at a frequency of either one or two times the rotational speed.